Investigating best practices for efficient minimal heating of high tunnels with modular heaters and row covers.

Progress report for FNC23-1384

Project Type: Farmer/Rancher
Funds awarded in 2023: $29,784.00
Projected End Date: 01/31/2025
Grant Recipient: Millsap Farms LLC
Region: North Central
State: Missouri
Project Coordinator:
James Millsap
Millsap Farms LLC
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Project Information

Description of operation:

Curtis and Sarah Millsap started Millsap Farms 16 years ago. They started with a 1/4 acre market garden, pastured poultry, and some nursery production. They now run a 25 acre farm, with 2.5 acres of production, 1/3 of which is under cover. They produce cut flowers, vegetables, and fruit, for a 225 member, 9-month CSA program, as well as a year-round farmers market, and some wholesale sales. They use organic practices, avoiding synthetic fertilizers, pesticides, and herbicides, and have transitioned over the years from a heavy tillage model to minimum tillage/regenerative agriculture model. They are deeply involved with their community, hosting weekly wood-fired pizza nights from May to October for 250 people a week, and monthly "twilight walks". Both of these community events include farm tours with farmer Curtis, to see the fields and tunnels, explain regenerative agriculture, and encourage others to go further in their own growing endeavors. They love to tell the story of sustainable agriculture, and are passionate advocates of appropriate technology for small scale farming, such as efficient pack sheds, effective use of high tunnels, monitoring systems, irrigation, etc.

Jason Hirtz took part in his first SARE funded project at Ivan Stoilov’s Fig Farm in Dittmer, Missouri in 2007. Box Turtle Farm established in 2008 as a CSA. In 2011, his family relocated to a 16-acre farm in Mount Vernon, Missouri. Box Turtle Farm began marketing to grocery stores and restaurants, eventually expanding to over 16,000 square feet of high tunnels. In 2013, the farm obtained USDA organic certification. Since 2012, Box Turtle Farm has produced salad through winter. their first efforts were in temporary caterpillar tunnels. In 2016, they expanded that idea by growing bunching greens and cucumbers in the summer through a SARE funded project titled “Evaluation of Alternative Coverings for Year Long Utilization of Caterpillar Tunnels”. In 2020-2021, they conducted another SARE funded study, "Biosolarization For Disease and Weed Control In Winter High tunnels." This project demonstrated that biosolarization was effective to control disease and weed control in high tunnels. Jason has shared his knowledge by speaking on the subject at several conferences including the Great Plains Growers Conference, the Midwest Winter Production Conference, Frozen Ground, numerous online Zoom appearances and the Thriving Farmer Podcast.


As our farms and many others have increased vegetable production in winter with unheated high tunnels, we’ve also increased our risk and liability. Even with the use of multiple layers of frost blankets within tunnels, crops are still damaged or killed by increasingly erratic weather. Poor air circulation and moisture buildup under the frost blankets also lead to crop diseases. It is no longer unusual in our region to experience temperature swings of 60 degrees or more in a 48 hour period, or a polar vortex that plunges temperatures to record lows (for example, the 16-18th of October we went from an 81 degree day to a 17 degree night 48 hours later). To insure against these losses, many farmers add heaters for emergencies, but because they are adding heat to high tunnels not set up for heating, and not interested in converting their tunnels to full greenhouses, the back up heat is inefficient. For farmers dedicated to sustainable farming, these inefficiencies are unacceptable. Using large amounts of fossil fuels to grow vegetables out of season is antithetical to their ideals. Many of us feel stuck between unacceptable crop losses in our tunnels and converting to full greenhouse production.

Project Objectives:


Some commercial greenhouses make use of an automated heat retention curtain that is deployed as an extra layer of insulation. This equipment is far too expensive for soil based high tunnel farmer, whose square foot sales are much lower. However, similarly, everyone farming with unheated tunnels is using row covers as frost blankets. On freezing nights, the blankets are pulled over the crops. The blankets trap some of the heat stored in the soil to help protect the crops during winter, a low-tech but high labor method.

We propose combining these blankets with portable, modular, unvented (hence extremely efficient) greenhouse heaters, to increase the effectiveness of the freeze protection and the efficiency of the heater in a high tunnel. Convection tubing is an inexpensive ducting made from clear, perforated 4mil plastic used to distribute heat in greenhouses. It is typically hung from the greenhouse trusses or laid under the canopy of the plants. This project would make use of this tubing to deliver heat from an LB White direct fired 170,000 BTU propane greenhouse forced air heater. Two convection tubes will be used in each house and will be set near the outer edges of the tunnel. Two farms are participating and each will install a heating system in two of their tunnels. One tunnel will use a frost blanket over the convection tubes and crop with the heater’s thermostat installed under the cover. The other tunnel will use the convection tubes and no frost blanket with its thermostat near the crop. Thermostats will be set at 35 degrees F. Temperature sensors will be placed throughout the tunnels.

The goal is to keep the crop above freezing temperatures as fuel efficiently as possible, and determine if the combination of the heat distributed under a frost blanket is more efficient than without. On freezing winter days, the routine of the farmer is the same; we go out in the afternoon and pull a frost blanket over our high tunnel crops before dark, and then remove the cover in the morning as the sun warms the structures. The covers used in this experiment will be Agribon Pro-50. One large piece will cover the entire growing space. At night when temperatures under the blanket drop below 35 degrees, a thermostat under the blanket will turn on the heater, blowing warm air under the cover and down the length of the tunnel via the convection tubing, ensuring there is no crop loss. The anticipated fuel savings is better for the environment, better for the farmer’s budget, and makes this crop insurance more palatable to the sustainable farmer. 

To make this whole system easily reproducible, we propose to use all off the shelf components, and heaters which are meant to be moved around as the need arises.  These heaters need no combustion air intake, and no exhaust, meaning that installation is as easy as wheeling them into place, and connecting electric, gas, and ductwork. This should make the system very adaptable and practical for farmers.


 1. Design and prototype two effective, reasonably priced systems for minimal heating (35 degrees F) of 30'x96' high tunnels.

2. Measure and quantify fuel and labor requirements of two different systems, one with row cover, one without. Produce usable data to guide other farmers who might want to adopt similar systems. 

3. Determine economic viability of these approaches, sharing that data in several field days, conference presentations, farm tours, and a regional farm gathering in spring of 2024. 

4. Produce and post to YouTube a video describing the systems an results, to further the quest for climate resilient farming practices.


Click linked name(s) to expand/collapse or show everyone's info
  • Jason Hirtz - Producer
  • James Curtis Millsap - Producer


Materials and methods:

We have assembled the heating units, based on the LB white tent heaters, with flexible duct work across the ends of the tunnels, and poly tube ducting down the aisles.  Propane tanks are set, filled, and being monitored with wireless monitors. Thermostats are in place, under row cover in the houses with cover, at plant canopy level in all houses.  In short, we're ready to start heating and have tested the systems on few cold nights over the past month. We had an unusually mild December, so the heating requirements were minimal.

Observations so far:

We purchased flexible ducting to connect the heater to the poly, because we wanted the flexibility. However, I think the price we pay with flexibility is decreased velocity and flow, which translates into underinflating the poly duct.  The warm air is still reaching the end of the duct, but they are not as taut as we were expecting. When we use overhead irrigation, they tend to not have the air pressure to dump the water off the ducts later. Another factor in this may be the spacing of our holes in the ductwork; we purchase ducts with a single row of holes punched 12" apart, but that might be too many holes.  If we were to start over, I would order half as many holes.

Research results and discussion:

So far, we have just achieved set-up, which took more time than we might have anticipated, mostly because one heater arrived damaged, and had to be serviced before we could set everything up. Nonetheless, we learned several important lessons:

  1. Getting propane companies to fill mobile tanks can be tricky; Boxturtle had no problems, but Millsap Farm had problems getting a company to come fill them.  Several providers declined to fill these smaller (100 gallon) tanks;
  2. Choose ductwork with fewer holes, perhaps holes every 24 inches, rather than 12; 
  3. The OEM thermostats provided with the heaters are robust, but not particularly precise. We are glad we had budgeted to purchase Inkbird thermostats.  As an added bonus, since we wrote the original proposal, Inkbird started selling a similarly priced unit with WIFI capability, which has allowed Millsap Farms to monitor and adjust temperatures in their tunnels with the app, an added feature we've really appreciated and; 
  4. Millsap is also loving the wireless tank monitors provided by their propane provider. The cost is $30/month for all three tanks on farm, and gives instantaneous readings of level, along with notification of low levels, and can be set to auto-request a refill at a given percentage.
Participation Summary
2 Farmers participating in research

Educational & Outreach Activities

1 Consultations
1 Tours

Participation Summary:

1 Farmers participated
1 Ag professionals participated
Education/outreach description:

Because we have only assembled the units, we have not hosted field days or other educational opportunities yet. We did have a visit from one NRCS representative, who came for a general farm tour, and had the opportunity to preview the heating system.  

Learning Outcomes

Lessons Learned:

See Materials and Research methods for results so far. 

Project Outcomes

2 Farmers changed or adopted a practice
2 New working collaborations
Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.